The original reason was that the propeller-driven aircraft prevalent when carriers were being developed tended to pull to port (through torque effect). Sticking the island to starboard prevented the buffer having to spend time scraping wafus off the house-side and reducing the ship husbandry (painting) burden. The island is to one side as it allows better use of the flightdeck and hangar areas.
Over time, it became a convention (much like driving on the right in uncivilised countries) and so interlinked with landing patterns that to change it would have caused more grief than it was worth. The japanese did build two with a port side island, but I believe they experienced one or two difficulties.